Interpretive Summary: Invasive plants are an increasing global problem that negatively affect our environment and many agricultural systems. Classical biological control, which involves the introduction of host specific herbivores to attack the target weed and bring it under natural control, is an increasingly important tool to help manage exotic invasive weeds. Eriophyid mites are extremely small and difficult to study; however, most are highly host specific. Research shows that there are many undiscovered species, and new species have been discovered on many weeds targeted for biological control. Therefore this group of mites has been considered to be an underutilized source of biological control agents. We reviewed recent and ongoing research using these mites for classical biological control of weeds. There have been several notable successes; however, other projects have revealed limitations and obstacles that must be overcome to acheive acceptable levels of control. We identify areas where future research is needed to help improve our understanding of these mites to enable us to use them more effectively.

Technical Abstract:
Eriophyid mites have been considered to have a high potential for use as classical biological control agents of weeds. However, in the past 20 years few species have been authorized for introduction, and few have significantly reduced the target plant's population. Natural enemies, resistant plant genotypes, and adverse abiotic conditions may all reduce the ability of eriophyid mites to control weeds. Furthermore, host specificity experiments conducted under laboratory conditions sometimes indicate a wider host range than that observed in the field, which results in failure to obtain approval for release. We need to know more about the natural behavior, life history and evolutionary stability of eriophyid mites. This is critical for designing and interpreting experiments to measure host plant specificity and potential impact on target and nontarget plants, which must be known before they can be released.